[Halld-controls] New resistors for QD, coil tap overcurrent protection and other QD modifications

[Elliott Wolin]

Hi,

In my previous note I reported a max 33% inductance imbalance based on 
Floyd's calculations and the hookup strategy we discussed previously.  
To be safe I propose we modify the QD to handle a 50% imbalance.  We 
should modify all four channels the same way so we don't tie any one 
channel to any particular set of coils.

Recall that the current-limiting resistor must be taken into account 
when modifying the QD.  To accommodate an inductance ratio of 1.5 I 
propose we replace all the 100k (1%) resistors in the outer divider 
chain with 20k (1%) resistors, along with the 100k resistor used for 
overcurrent protection.  Including the overcurrent resistor there would 
be five 20k resistors on each side of a 50k pot (POT2), or 100k + 50k + 
100k total on the outer divider chain.  With the pot set to the extreme 
ends the maximum divider ratio would be 150/100 = 1.5.

I will check with Alexander Elkiaer, but probably the inner 100k divider 
chain resistors should be changed as well, although they do not directly 
affect the imbalance compensation.  Also, I'll ask him if the cable 
disconnect resistors need to be modified.

An important question is whether 20k is large enough enough for 
overcurrent protection.  If not we may have to modify POT2 as well.

Mark...what are your thoughts concerning using 20k overcurrent 
protection resistors?

Finally, I believe we should bring -IN out to a test point on the back 
panel to allow us to zero-out Fernando's sensitivity asymmetry 
compensation circuit.  I think this needs to be zero'd out as part of 
the full QD adjustment procedure prior to setting the bias pot (POT2), 
after which his circuit can be set to compensate for the sensitivity 
asymmetry caused by the inductance imbalance.  Maybe this isn't 
absolutely necessary, but it may make life easier and it is simple to add.

Thoughts, comments...?

Thanks,

-- 

				Sincerely,
					Elliott


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  Those raised in a morally relative or neutral environment will hold
		    no truths to be self-evident.
				

Elliott Wolin
Staff Physicist, Jefferson Lab
12000 Jefferson Ave
Suite 8 MS 12A1
Newport News, VA 23606
757-269-7365

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